sunlight - these plants need full, or a good number of hours, of sun to keep their purple colour, like most purple leaved plants.

Did you move your plant to another location? If you did, try moving it back or to another place where it has adequate sun.

Mine goes greenish in winter when we get less light. It's colour returns as the days lengthen.
Try to keep it in a south facing window in winter, so that it gets as much light as possible.

Thanks for the information everyone

You're welcome !